| 1. |
- Bergqvist, Lars, 1976-, et al.
(författare)
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Magnetic and electronic structure of (Ga1-xMnx)As
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:20, s. 205201-
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Tidskriftsartikel (refereegranskat)abstract
- We present theoretical calculations of the magnetic and electronic structure of Mn-doped GaAs (in the zinc-blende structure). The magnetic properties are shown to be very sensitive to structural defects, in particular, As antisite defects and Mn at interstitial positions. Only when considering such defects can the experimental magnetic moments be reproduced by first-principles theory. We present a simple model for understanding the connection between the magnetic ordering and the As antisites, and the way in which the defects help to stabilize a partial disordered local-moment state. The connection between the energetics of the Mn substitution and the As antisite concentration is also analyzed. In addition, we compare the calculated magnetic properties and electronic structures of Mn situated on substitutional sites (Mn replacing a Ga atom) and on interstitial sites, where in agreement with observations the interstitial site is found to be less favorable. Finally, combining our first-principles calculations of the spin-wave excitation energies with a classical Heisenberg Hamiltonian we have calculated interatomic exchange interactions, and using Monte Carlo simulations we present theoretical values of the critical temperature as a function of Mn concentration.
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| 2. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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Constitutional and thermal point defects in B2 NiAl
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:9, s. 6003-6018
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Tidskriftsartikel (refereegranskat)abstract
- The formation energies of point defects and the interaction energies of various defect pairs in NiAl are calculated from first principles within an order N, locally self-consistent Green's-function method in conjunction with multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground state for the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms and Ni vacancies in Ni-rich and Al-rich NiAl, respectively) are shown to form ordered structures in the ground state, in which they tend to avoid each other at the shortest distance on their sublattice. The dominant thermal defects in Ni-rich and stoichiometric NiAl are calculated to be triple defects. In Al-rich alloys another type of thermal defect dominates, where two Ni vacancies are replaced by one antisite Al atom. As a result, the vacancy concentration decreases with temperature in this region. The effective defect formation enthalpies for different concentration regions of NiAl are also obtained.
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| 3. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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Defect-induced magnetic structure in (Ga1-xMnx)As
- 2002
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 88:18, s. 187202-
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Tidskriftsartikel (refereegranskat)abstract
- We show that magnetic structures involving partial disorder of local magnetic moments on the Mn atoms in (Ga1-xMnx)As lower the total energy, compared to the case of perfect ferromagnetic ordering, when As defects on the Ga sublattice are present. Such magnetic structures are found to be stable for a range of concentrations of As antisites, and this result accounts for the observed magnetic moments and critical temperatures in (Ga1-xMnx)As . We propose an explanation for the stabilization of the partially disordered magnetic structures and conclude that the magnetization and critical temperatures should increase substantially by reducing the number of As antisite defects.
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| 4. |
- Smirnova, E. A., et al.
(författare)
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Electronic topological transitions and phase stability in the fcc Al-Zn alloys
- 2002
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 30:1, s. 57-66
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed investigation of the phase equilibria and the Fermi surface in the Al-Zn system. Our calculation are based on the density functional theory and we use the linear muffin-tin orbital method and the Green's function technique. The calculated free energies of alloy formation exhibit the existence of a miscibility gap between the alloys containing approximately 10 and 55 at.% of Zn, in agreement with the phase diagram of the Al-Zn system. Seven electronic topological transitions (ETT) were found in Al-Zn system within the stability range of the fcc solid solution. A relation between these ETT and the phase stability of the fcc Al-Zn solid solutions is established. We show that extremum points on the concentration dependencies of the thermodynamic properties of Al-Zn alloys can be explained by band-filling effects.
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| 5. |
- Smirnova, E. A., et al.
(författare)
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Origin of the asymmetric spinodal decomposition in the Al-Zn system
- 2001
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 64:2, s. 201011-201014
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Tidskriftsartikel (refereegranskat)abstract
- The Fermi surface and the phase equilibria in the Al-Zn system are studied theoretically in the framework of the density-functional theory, linear muffin-tin orbital method, and the Green's function technique. Analyzing the Fermi surfaces of the random alloys, we find a series of electronic topological transitions within the stability range of the fcc solid solution. A relation between these transitions and the phase stabilities of fee Al-Zn is established. We show that the asymmetry of the miscibility gap as well as of the mixing energy are due to the electronic topological transitions in the system.
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| 6. |
- Korzhavyi, Pavel A., et al.
(författare)
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Oxidation of plutonium dioxide
- 2004
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Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 3:4, s. 225-228
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Tidskriftsartikel (refereegranskat)abstract
- The physics and chemistry of the actinide elements form the scientific basis for rational handling of nuclear materials(1-3). In recent experiments(4), most unexpectedly, plutonium dioxide has been found to react with water to form higher oxides up to PuO2.27, whereas PuO2 had always been thought to be the highest stable oxide of plutonium(2,3). We perform a theoretical analysis of this complicated situation on the basis of total energies calculated within density functional theory(5,6) combined with well-established thermodynamic data. The reactions of PuO2 with either O-2 or H2O to form PuO2+delta are calculated to be endothermic: that is, in order to occur they require a supply of energy. However, our calculations show that PuO2+delta can be formed, as an intermediate product, by reactions with the products of radiolysis of water, such as H2O2.
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| 7. |
- Ambrosch-Draxl, C., et al.
(författare)
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First-principles study of oxygen ordering in YBa2Cu3O7-x
- 2000
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 341, s. 1997-1998
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Tidskriftsartikel (refereegranskat)abstract
- We have performed first-principles calculations considering different patterns of oxygen/vacancy ordering in the Cu-O chains of YBa2Cu3O7-x. From the total-energy values we determine the effective interactions of the oxygen atoms within the basal plane by using the cluster expansion technique. Furthermore, we discuss the effect of ordering to the density of states at the Fermi level.
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| 8. |
- Eriksson, Olle, et al.
(författare)
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Electronic structure and magnetism of diluted magnetic semiconductors
- 2004
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 16:48, s. S5481-S5489
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and magnetism of selected diluted magnetic semiconductors (DMS) is reviewed. It is argued that the effect of antisite defects plays an important role in the magnetism of DMS materials and that these defects lower the saturation moment and ordering temperature. We also show that the interatomic exchange of these materials is short ranged. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we show that-the observed critical temperatures of a broad range of diluted magnetic semiconductors, involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, are reproduced with good accuracy. We show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned on the Ga (or Zn) sites. This suggests that the ordering of DMS materials is heavily influenced by magnetic percolation and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.
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| 9. |
- Hugosson, Håkan Wilhelm, et al.
(författare)
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Phase stabilities and structural relaxations in substoichiometric TiC1-x
- 2001
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 6316:16, s. 165116-
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations have been used to study the effect of vacancies and relaxation around the vacancy sites in substoichiometric TiC1-x. The effect of relaxation on phase stabilities, equilibrium volumes. and electronic structure of the substoichiometric phases was studied using a combined approach of pseudopotential plane wave and full-potential linear muffin-tin orbital methods. A relaxation away from the vacancies was found for the titanium atoms, the magnitude of which increased with vacancy concentration and the inclusion of nearest-neighbor carbon atom relaxation. The inclusion of local relaxations was found to correctly predict the off-stoichiometric equilibrium composition of titanium carbide. The anomalous volume behavior of TiC at small vacancy concentration is explained as an effect of the local relaxation of the atoms surrounding the vacancy sites, but we do not find that the lattice parameter of any of the studied stoichiometries is larger than that of ideal stoichiometric TiC.
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| 10. |
- Isaev, Eyvaz, et al.
(författare)
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Intrinsic defects and transition metal impurities in GaAs
- 2004
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 272:3, s. 1961-1962
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Tidskriftsartikel (refereegranskat)abstract
- Thermodynamics of intrinsic point defects, 3d-transition metal (TM) impurities, and various defect pairs in GaAs have been studied by means of the locally self-consistent Green's function method. Antisite defects, As-Ga, are found to be the most energetically favorable defects in the As-rich GaAs. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The magnetic moments of TM impurities in the GaAs host are calculated under the assumption that orbital moments of TM are quenched and the total magnetization is due to spill ordering of electrons in the crystal. Thermodynamic possibility of formation of complexes between TM-atoms is investigated.
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| 11. |
- Johansson, Börje, et al.
(författare)
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Chemical composition-elastic property maps of austenitic stainless steels
- 2003
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Ingår i: Solid State Sciences. - 1293-2558 .- 1873-3085. ; 5:6, s. 931-936
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Tidskriftsartikel (refereegranskat)abstract
- Despite a tremendous development during the last decades, both as regards computer power and methodology, it has remained impossible to treat steel at a fundamental atomic level. However, recently we have shown [L. Vitos, P.A. Korzhavyi, B. Johansson, Phys. Rev. Lett. 88 (2002) 155501] that the most efficient theories of random substitutional alloys combined with advanced numerical techniques have made possible to establish a theoretical insight to the electronic structure of stainless steels. Here a detailed description of the quantum-mechanical modeling of austenitic stainless steels is presented. We adopt an ab initio electronic structure calculation method based on the coherent potential approximation, implemented within the framework of the exact muffin tin orbitals theory, to map the chemical composition distributions of austenitic stainless steels into the elastic property distributions. The so generated database can be fruitfully used in the design of new class of steel alloys.
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| 12. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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Ab Initio Study of Phase Equilibria in TiCx
- 2002
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Ingår i: Physical Review Letters. - : The American Physical Society. - 0031-9007 .- 1079-7114. ; 88:1, s. 015505-1-01505-4
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Tidskriftsartikel (refereegranskat)abstract
- The phase diagram for the vacancy-ordered structures in the substoichiometric TiCx (x=0.5-1.0) has been established from Monte Carlo simulations with the long-range pair and multisite effective interactions obtained from ab initio calculations. Three ordered superstructures of vacancies (Ti2C, Ti3C2, and Ti6C5) are found to be ground state configurations. Their stability has been verified by full-potential total energy calculations of the fully relaxed structures.
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| 13. |
- Korzhavyi, Pavel A., et al.
(författare)
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Thermodynamic properties of Pu-O-H compounds and alloys from density functional theory
- 2004
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Ingår i: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXVII. - WARRENDALE : Materials Research Society. - 1558997520 ; , s. 107-112
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Konferensbidrag (refereegranskat)abstract
- A theoretical approach has been developed that allows one to obtain thermodynamic properties of plutonium-based alloys and compounds from first-principles electronic structure calculations based on density functional theory. The approach is applied to study the defect structure in non-stoichiometric PuO2+/-delta.
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| 14. |
- Ruban, Andrei V., et al.
(författare)
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Screened Coulomb interactions in metallic alloys. II. Screening beyond the single-site and atomic-sphere approximations
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:2
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Tidskriftsartikel (refereegranskat)abstract
- A quantitative description of the configurational part of the total energy of metallic alloys with substantial atomic size difference cannot be achieved in the atomic-sphere approximation: It needs to be corrected at least for the multipole-moment interactions in the Madelung part of the one-electron potential and energy. In the case of a random alloy such interactions can be accounted for only by lifting the atomic-sphere and single-site approximations, in order to include the polarization due to local environment effects. Nevertheless, a simple parametrization of the screened Coulomb interactions for the ordinary single-site methods, including the generalized perturbation method, is still possible. We obtained such a parametrization for bulk and surface NiPt alloys, which allows one to obtain quantitatively accurate effective interactions in this system.
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| 15. |
- Vitos, Levente, et al.
(författare)
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Austenitic stainless steels from quantum mechanical calculations
- 2004
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Ingår i: Advanced Engineering Materials. - : Wiley. - 1438-1656 .- 1527-2648. ; 6:4, s. 228-232
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Tidskriftsartikel (refereegranskat)abstract
- Quantum mechanics is used to study the influence of the chemical composition on the elastic properties of austenitic stainless steels. Fe based alloys comprising approximately 15% Cr and 8% Ni are predicted to have the largest hardness among the usual austenitic steels, which, however, is associated with increased brittleness and susceptibility to various forms of localized corrosion. It is shown that few percent of additional Os or Ir to Fe15Cr8Ni alloy significantly improve on both of these shortcomings, without deteriorating the hardness.
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| 16. |
- Vitos, Levente, et al.
(författare)
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Elastic property maps of austenitic stainless steels
- 2002
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 88:15, s. 155501-
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Tidskriftsartikel (refereegranskat)abstract
- The most recent advances in theory and methodology are directed towards obtaining a quantitative description of the electronic structure and physical properties of alloy steels. Specifically, we employ ab initio alloy theories to map the elastic properties of austenitic stainless steels as a function of chemical composition. The so generated data can be used in the search for new steel grades, and, as an example, we predict two basic compositions with outstanding properties among the austenitic stainless steels.
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| 17. |
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